1. Field of the Invention
The present invention relates to heat blankets and more particularly to an internally temperature regulated heat blanket.
2. Description of the Prior Art
Heretofore, the problem with existing heat blankets is that they do not provide a safe, uniform temperature when covering non-uniform cold areas or heat sinks having variable heat transfer characteristics. Current blankets generally utilize some form of electrical resistance wire, such as inconel, balco, or nichrome wire as a heating element. Another problem with current blankets is that the wires can be broken during flexing or application of the blanket to existing heaters using a contoured surface.
It is also possible for thermal overshoot (excessive temperature excursions) to occur in the presence of variable heat sinks and high thermally resistant insulation surrounding the heat source. The thermal overshoot is not desired since it can cause damage to the surrounding area or may even destroy composite materials undergoing repair. This problem occurs because the heating element is a high temperature source for the required thermal energy, and during rapid restoration of heat following depletion, the system overshoots the set surface temperature. This can therefore require that resistance wire types of heaters incorporate thermocouples, hot bond regulators, and computers to monitor and control safely the total overall temperature of the blanket.
Other technologies for heat application for composite repair on aircraft can be complex and may include the use of components such as metals excited by high frequency RF, or xe2x80x9cloadedxe2x80x9d polymers of conductive material that, when similarly energized, provide a given heat for their designed configuration.
The prior art patent literature includes:
U.S. Pat. No. 4,937,435 to Goss et al., which discloses a flexible electric heating pad using positive temperature coefficient (PTC) ceramic thermistor chip heating elements. As shown in FIG. 1, the pad space P has thermistors 10 inserted into separating dielectric insulator 12. Conductive sheets 16 and 18 are provided parallel to each other on opposite sides of the dielectric 12. Insulating layer 20 is provided to protect the heating pad P from the environment. The metallic sheet 22 may be formed over the insulating layer 20. Conductors 17 and 19 can be attached to the conductive sheets 16 and 18.
U.S. Pat. No. 4,177,376 to Horsma et al. (positive temperature coefficient) which illustrates a layered self-regulating heat article in which a PTC layer 49 is provided between a layer of constant wattage material 47 having electrodes 48 embedded therein and a second constant wattage layer 50 with electrodes 51 embedded within. Insulation layers 46 and 53 are provided outside of layers 47 and 50, respectively.
U.S. Pat. No. 4,684,785 to Cole which teaches an electric blanket having a heating element with at least two electrodes separated by a heating material with a positive temperature coefficient of resistance.
U.S. Pat. No. 4,761,541 to Batliwalla et al. which relates to a device comprising conductive polymer compositions and has a laminar PTC conductive polymer element 11 on one surface of an electrode 12. Electrodes 13 and 14 are separated from the electrode 12 by the PTC electrode 11.
U.S. Pat. No. 4,733,057 to Stanzel et al. which teaches a sheet heater which includes multiple self-regulating PTC conductive polymer heater elements which are disposed parallel to one another and held in place by supports of rigid polymeric material such as polyamide.
It is an object of the present invention to overcome the limitations of the prior designs by providing a heat blanket utilizing a plurality of PTC devices arranged to have a high surface area utilization to eliminate hot spots; and which additionally features low thermal resistance transfer paths to prevent overshooting of the intended temperature range.
A design to which the electrical interconnects of the heating element serve also as a heat transfer device which is not found in present blanket construction. The physical arrangement of the electrical interconnects make it possible to repair a defective heating element if necessary rather than rendering the entire blanket defective, which is the case with present blankets.
It is yet another object of the present invention to provide a heat blanket design utilizing positive temperature coefficient devices as stable heating elements which will not overshoot their intended temperature range; which heat blanket may be cut into another geometry without destroying or compromising heat transfer.
The invention is a heat blanket for cure of composite parts or to other items such as food carts or trays that require stable heat sources and uniform application of heat. The blanket is composed of an outer layer of fiberglass for mechanical protection, a layer of closed cell silicone foam for thermal and electrical insulation, a layer of thermally conductive but electrically insulating silicone product, a layer of electrically conductive mesh, another layer of thermally conductive silicone with holes cut into it in which are placed positive temperature coefficient (PTC) heating elements, another layer of conductive mesh, a layer of thermally conductive silicone and an inner layer of moderately conductive cured silicone or foam. The positive temperature coefficient elements will maintain a constant temperature as long as sufficient current is available. The two layers of conductive mesh form the electrical connections for the heating elements. Optimally, there is a strip of foil around the perimeter of each layer of conductive mesh to provide relatively easy electrical connections. The blanket may be cut to any shape or size, although cutting the (PTC) heating elements is difficult unless they are very thin. The use of PTC heating elements eliminates the need for sophisticated temperature control. In practice, the blanket can maintain 350 degrees Fahrenheit on the inside and still allow physical contact on the outside without burning the operator. The outer layer of the heat blanket is described in Boeing U.S. Pat. No. 5,330,809 and provides a thermal barrier and flame retardant benefits. The flame retardant characteristics of the top layer of foam provides a self-extinguishing feature to a heating element that may destruct in operation by thermal runaway that can cause temperatures to exceed to greater than the design characteristics of the PTC device.